The present invention relates to aqueous medium treatment processes and apparatus, which can be used to treat organic wastewater, synthesize a product or recover a metal by efficiently performing hydrothermal reaction and electrolysis at the same time. As used herein, hydrothermal electrolysis means that hydrothermal reaction and electrolysis are performed at the same time.
Various kinds of waste liquors have been conventionally treated by hydrothermal reaction. Hydrothermal reaction means that an aqueous medium such as waste liquor is exposed to a pressure that allows said aqueous medium to be kept in the liquid phase at high temperature below the critical temperature of the aqueous medium, whereby reducing substances such as organics are degraded at high temperature.
However, waste liquors could not be treated at sufficient efficiency by conventional hydrothermal reaction processes.
Thus, we proposed an efficient waste liquor treatment process by hydrothermal electrolysis (International Application PCT/JP 98/03544 filed Aug. 10, 1998; see International Publication WO99/07641). Said hydrothermal electrolysis is a process for effectively oxidatively degrading reducing substances such as organics (including synthetic polymers) or ammonia by performing hydrothermal reaction and electrolysis at the same time in the presence of water at high temperature and high pressure. The disclosure of International Publication WO99/07641 is incorporated herein as a whole as reference.
Although this hydrothermal electrolytic reaction is useful as a process for very effectively degrading reducing substances, later studies revealed that a large electricity is required for the treatment of waste liquor. That is, it is necessary to increase the electrolytic current, and therefore to increase the area of electrolytic electrodes in order to continuously and rapidly treat a large amount of waste liquor by hydrothermal electrolytic reaction. However, it is not always easy to increase the area of electrodes in a reaction vessel, which should be exposed to high temperature and high pressure of hydrothermal electrolytic reaction. Therefore, how to increase the electrolytic current in a limited electrode area was a great issue for increasing the throughput in said treatment process.
Possible electrode reactions that can proceed in hydrothermal electrolytic reaction are described below. However, the present invention is not bound to the theory described below. At the anode, reactions (1), (2), (3) below seem to proceed.
2O2xe2x88x92xe2x86x92O2↑+4exe2x88x92xe2x80x83xe2x80x83(1)
H2Oxe2x86x922H++1/202↑+2exe2x88x92xe2x80x83xe2x80x83(2)
Organic+H2Oxe2x86x92CO2↑+H++exe2x88x92xe2x80x83xe2x80x83(3)
When a halide ion exists in the aqueous medium, a halogen molecule is produced by the formula below.
2Xxe2x88x92xe2x86x92X2+2exe2x88x92xe2x80x83xe2x80x83(4)
where X represents a halogen atom.
In formula (1), the molecular oxygen produced serves as an oxidizer. In formula (1), a very active chemical species such as atomic oxygen seems to be produced as the molecular oxygen is produced at the interface between the anode and the electrolyte. In formula (4), a halide ion is oxidized to produce a halogen molecule. When X is a chlorine atom, for example, chlorine gas is produced. In formula (2), water is electrolyzed to produce oxygen gas. In formula (3), an organic is directly oxidized at the anode. The reaction of formula (4) and the reaction of formula (2) compete with each other and which reaction prevails depends on the type of the anode, the halide ion concentration in the aqueous medium and other factors. For example, the reaction of formula (4) prevails when a chlorine-generating electrode is used at a specific halide ion concentration or more.
The halogen molecule produced at the interface between the anode and the electrolyte by formula (4) reacts with its neighboring water to produce a hypohalous acid and a hydrogen halide.
X2+H2Oxe2x86x92HX+HXOxe2x80x83xe2x80x83(5)
where X has the meaning as defined above.
Hypohalous acids are excellent oxidizers capable of oxidatively degrading reducing substances contained in aqueous media. When the reducing substance is an organic, for example, the organic seems to be oxidized by the reaction below.
xe2x80x83Organic+HXOxe2x86x92CO2↑+H2O HXxe2x80x83xe2x80x83(6)
where X has the meaning as defined above.
When the reducing substance is ammonia, ammonia seems to be oxidized by the reaction below.
2NH3+3HXOxe2x86x92N2↑+3HX+3H2Oxe2x80x83xe2x80x83(7)
Hypohalous acids are excellent oxidizers especially in acidic solutions and hydrogen ion is produced by formulae (2), (3) or the like to tend to form an acidic environment near the anode at which a hypohalous acid is produced. Thus, the hypohalous acid seems to especially favorably act as an oxidizer near the anode.
When X is a chlorine atom, the oxidation reaction by the hypohalous acid seems to especially participate in the degradation of reducing substances.
When X is a bromine atom or an iodine atom, the halate ion may participate in the degradation of reducing substances. Hypohalite ions disproportionate in basic solutions to produce a halate ion and a halide ion.
3XOxe2x88x92xe2x86x922Xxe2x88x92+XO3xe2x88x92xe2x80x83xe2x80x83(8)
For example, the reaction of formula (8) may occur when the hypohalous acid moves toward the cathode by diffusion or the like. This is because hydroxide ion is produced by anodic reaction to tend to form a basic environment near the cathode. The rate of the disproportionation reaction of formula (8) is higher in the order of chlorine, bromine and iodine, and a halate ion can be quantitatively obtained with bromine and iodine (F. A. Cotton, G. Wilkinson, P. L. Gaus, xe2x80x9cBasic Inorganic Chemistryxe2x80x9d, Baifukan, 1991, 2nd ed., p. 379). Halic acids are strong acids and potent oxidizers.
In formula (2), oxygen gas is produced by the electrolysis of water. Here, an oxygen atom seems to be first produced at the interface between the anode and the electrolyte. Said oxygen atom has a higher activity as an oxidizer than molecular oxygen so that it can efficiently oxidize reducing substances. Even if molecular oxygen is produced, reducing substances can be oxidized by hydrothermal oxidation reaction.
When the reducible substance is an organic, oxidation reaction by oxygen proceeds by the formula below.
Organic+O2xe2x86x92CO2↑+H2Oxe2x80x83xe2x80x83(9)
As shown by formula (3), reducing substances such as organics or ammonia may be directly oxidized at the anode by electrode reaction. When the reducing substance is ammonia, for example, the reaction of the formula below may proceed.
2NH3xe2x86x92N2↑+6H++6exe2x88x92xe2x80x83xe2x80x83(10)
Thus, hydrothermal electrolysis according to the present invention includes many reaction mechanisms by which reducing substances are efficiently oxidatively degraded at or near the anode.
On the other hand, possible reactions at the cathode are as follows.
Water is electrolyzed to produce hydrogen at the cathode.
2H2O+2exe2x88x92xe2x86x92H2↑+2OHxe2x88x92xe2x80x83xe2x80x83(11)
Here, the so-called cathodic protection against corrosion can be provided by using the reactor body as a cathode.
A reaction may also proceed in which an oxidizer is reduced at the cathode. The oxidizer here includes an oxidizer produced at the anode such as a hypohalous acid and optionally an externally added oxidizer. Examples of reaction are shown by formulae (12), (13), (14) and (15) below.
The hypohalous acid is reduced at the cathode.
HXO+exe2x88x92xe2x86x92Xxe2x88x92+OHxe2x88x92xe2x80x83xe2x80x83(12)
Oxygen dissolved in the aqueous medium represented by O2(aq) in the formulae below is also reduced.
1/2O2(aq)+H2O+2exe2x88x92xe2x86x922OHxe2x88x92xe2x80x83xe2x80x83(13)
Another possible reaction of cathodic reduction of dissolved oxygen is as follows.
O2(aq)+H2O+exe2x88x92xe2x86x92active oxygen+OHxe2x88x92xe2x80x83xe2x80x83(14)
If hydrogen peroxide exists, it is reduced at the cathode.
H2O2+2exe2x88x92xe2x86x922OHxe2x88x92xe2x80x83xe2x80x83(15)
At the cathode, the reactions of formulae (12), (13), (14) and (15) in which an oxidizer is reduced compete with the reaction of formula (11) in which hydrogen is generated.
Our experiments revealed that the reactions of formulae (12), (13), (14) and (15) in which an oxidizer is reduced proceed preferentially to the reaction in which hydrogen is generated in hydrothermal electrolysis, and especially the reaction of formula (14) in which active oxygen is produced actively proceeds. Correspondingly, hydrogen generation is inhibited in hydrothermal electrolysis to reduce the possibility of coexistence of oxygen gas and hydrogen gas in the reactor and thus to reduce the danger of explosion. The oxidizer such as a hypohalous acid is degraded at the cathode to eliminate the secondary treatment for detoxifying the oxidizer in the effluent. For example, a hypohalite ion is generated at a high concentration during electrolysis at room temperature. However, the generation of a hypohalite ion was scarcely detected during electrolysis at high temperature.
It is thought that reducing substances such as organics or ammonia are oxidatively degraded by hydrothermal electrolytic reaction according to the reaction mechanisms as described above to inhibit the generation of hydrogen gas or oxygen gas.
Thus, hydrothermal electrolytic reaction involves electrolyzing an aqueous medium containing water, a halide ion such as chloride ion and reducing substances such as organics or ammonia at specific high temperature and high pressure to oxidatively degrade the reducing substances. In electrolysis, oxidation reaction proceeds at the anode to produce oxygen gas and a halogenic oxidizer such as a hypohalous acid. Generally, oxidation reaction readily proceeds in the presence of an oxidizer such as oxygen gas at high temperature and high pressure of hydrothermal reaction. In hydrothermal electrolytic reaction according to the process of the present invention, reducing substances such as organics or ammonia can be effectively oxidatively degraded by performing hydrothermal reaction and electrolysis at the same time.
Moreover, persistent substances could be treated at high degradation efficiency with a nascent oxidizer produced in situ in the hydrothermal electrolytic reactor such as an active species produced during the generation of molecular oxygen in formula (1). When an external oxidizer such as oxygen in the air was directly introduced into the reaction site of this hydrothermal electrolysis, this external oxidizer having low electrochemical activity could be converted into active oxygen having high activity to further improve the efficiency of this degradative reaction.
However, nascent oxidizers or active oxygen electrochemically produced in the reactor entail higher production costs as compared with normal oxidizers (external oxidizers), though they have high activity. The amount of internal oxidizers produced depends on the electricity so that enormous electricity is needed if reducing substances contained in an aqueous medium are wholly treated with internally produced oxidizers alone. On the contrary, external oxidizers, especially compressed air or the like can be introduced into the hydrothermal reaction site at low running costs involving only power costs for the motor of the compressor for compressing air.
Generally, aqueous media contain various reducing substances including persistent substances which are difficult to degrade with external low-activity oxidizers (air, oxygen, hydrogen peroxide, ozone, hypohalous acids, etc.) and readily degradable substances which are readily degradable with such external oxidizers. Said readily degradable substances include, for example, t-butyl alcohol, formic acid, oxalic acid, phenol, o-cresol and benzyl alcohol, and said persistent substances include, for example, acetic acid, propionic acid, succinic acid, adipic acid, propylene glycol and polyethylene glycol 200. Aqueous media contain ammonia and various organics that are degradable by electrolysis under normal temperature and atmospheric pressure conditions. When these readily degradable substances or electrolytically degradable ammonia or various organics exist at the hydrothermal electrolytic reaction site, they consume nascent oxidizers or active oxygen to increase power consumption of the hydrothermal electrolytic reactor. High-activity electrochemical oxidizers internally produced in hydrothermal electrolytic reaction are consumed to degrade not only persistent substances but also readily degradable substances or electrolytically degradable ammonia or various organics, thus increasing power consumption for hydrothermal electrolysis and therefore running costs of the aqueous medium treatment process.
Thus, an object of the present invention is to provide a process and an apparatus for treating an aqueous medium at low running costs with decreased electricity required for hydrothermal electrolytic reaction.
As a result of careful studies to attain the above object, we accomplished the present invention on the basis of the finding that a two-step process comprising a first step of performing a conventional hydrothermal reaction or a conventional electrolytic reaction prior to hydrothermal electrolytic reaction to degrade readily degradable substances or ammonia or various electrolytically degradable organics as described above and a-second step of performing hydrothermal electrolytic reaction to degrade the remaining organics is effective to remarkably reduce the electricity consumed by the hydrothermal electrolytic reaction in the second step.